The present invention relates to a mass spectrometer and a method of mass spectrometry.
Tandem mass spectrometry such as tandem quadrupole (“QqQ”) and quadrupole Time of Flight (“QToF”) mass spectrometers have proved to be an invaluable tool in many applications. Tandem quadrupole instruments, in particular, have found roles in screening applications using semi targeted analyses such as parent or precursor ion scans and neutral loss scans. These types of analyses typically involve fragmenting ions exiting a scanning mass to charge ratio mass filter and using a second mass to charge ratio mass filter to target a particular fragment ion or fragment loss. As the first step of mass analysis is via a scanning mass filter, the duty cycle, and consequently the sensitivity, is reduced depending on the resolution of the mass filter and the mass to charge ratio range scanned.
To a first approximation, and for illustrative purposes only, the transmission characteristics of a mass to charge ratio mass filter can be approximated as having a uniform profile of width W Da, wherein the value of W is related to the instrument resolution. If the mass to charge ratio range to be scanned is denoted as Rg Da and the mass filter scanning speed is denoted as Sp Da/s then the length of time spent scanning across any particular mass to charge ratio is given by W/Sp (s) and the time spent for the full range scan is given by Rg/Da (s). The duty cycle is therefore given by (W/Sp)/(Rg/Sp) which simplifies to W/Rg.
A particular disadvantage, therefore, of known scanning instruments such as quadrupole mass analysers is that they suffer from low duty cycle.
A method of Automatic Gain Control (“AGC”) is known which involves automatically controlling the number of ions entering a mass analyser by performing a pre-scan to determine the Total Ion Charge (“TIC”), An ion injection time is then calculated for the analytical scan based upon the determination of the Total Ion Charge. This approach prevents space charge saturation of the mass analyser.
It is desired to provide an improved mass spectrometer.